This invention is related to previous disclosures such as European Patent Application 85303807.3, Publication No. 0 164 248 directed to acid hardening resin containing photoresist compositions and processes for their use, as well as generally to conventional positive acting photoresist compositions.
A positive acting photoresist is a film forming composition which typically contains a film forming polymer and a photosensitive compound dissolved in a suitable solvent. The photoresist composition is applied to a substrate surface, such as for example the surface of a silicon wafer in the preparation of an integrated circuit or computer chip, as a film. A photomask is positioned between the substrate surface containing the photoresist film and a source of exposing radiation. The photomask acts like a stencil having portions which are transparent to the exposing radiation to be used and other portions which are opaque to the exposing radiation. The photomask is designed and fabricated to duplicate the electronic circuit pattern which is desired to be transferred onto the substrate. The exposing radiation to be employed is typically radiation of a single or narrow range of wavelengths, referred to as actinic radiation, in the ultraviolet or shorter wavelength spectra, such as for example x-rays and electron beams. The photoresist film is exposed to the selected exposing radiation at a predetermined dosage and time through the photomask. Through the proper positioning of the photomask relative to the photoresist film only those portions of the photoresist film juxtaposed to the transparent portions of the photomask are exposed to the actinic radiation. The exposing radiation absorbed by the photoactive compound in the photoresist film causes a chemical reaction in the exposed portions of the film. In the case of a positive acting photoresist this chemical reaction renders the exposed portions of the photoresist film more soluble than the unexposed portions of the film to certain selected solutions known as developers. This differential solubility between the exposed portions and unexposed portions of the photoresist film to such developers permits the selective removal of the exposed portions in a development step thereby leaving the unexposed portions of the photoresist film on the substrate. This creates a pattern for the subsequent deposition of a conductive metal, insulators or dopants, or for etching the substrate, in the portions of the substrate not protected by the remaining photoresist portions. These remaining photoresist portions are removed from the substrate after the deposition of the conductive metal circuitry by stripping the photoresist using selected stripping solvents.
In the above described process it is desirable for a positive photoresist to possess as wide a differential solubility as possible between the exposed and unexposed portions of the film as well as the ability of the unexposed portions to be as resistant as possible to the subsequent chemicals and conditions used in the subsequent processing.
In the case of negative acting photoresist compositions the development step selectively removes the unexposed portions of the photoresist film leaving the exposed portions of the photoresist film on the substrate. Accordingly if one desires to produce the same circuit pattern on a substrate using a negative acting photoresist composition as described above as with a positive acting photoresist composition generally the type of photosensitive compound, the process and the photomask need to be modified. In the case of the photomask the portions of the positive photomask which are transparent to the exposing radiation need to be made non transparent or opaque to the exposing radiation and those portions of the positive photomask which are opaque to the exposing radiation need to be transparent to the exposing radiation. Similarly the photosensitive compound used in the photoresist composition needs to be selected such that the exposed portions of the photoresist film are less soluble in the selected developer solution than the unexposed photoresist film portions.
At the present time the majority of the high resolution commercial photoresist compositions are based on positive acting chemistry and processes, although recent advances in the application of acid hardening chemistry to high resolution photoresists has spurred renewed interest in negative acting photoresists. Acid hardening chemistry in negative acting photoresists offers the advantage of creating high resolution, crosslinked polymeric images on substrates. These crosslinked images not only offer improved chemical and thermal resistance during subsequent processing of the substrate, but due to the increased differential solubility between the exposed and unexposed photoresist film portions permit wider processing latitude for the development step.
Acid hardening chemistry employs the combination of an acid hardening resin system and an acid which catalyzes the crosslinking of the acid hardening resin system in the photoresist when the photoresist film is heated.